1. Field of the Invention
The present invention relates to the field of production of steel, more specifically to a method of making steel with reduced internal stress concentrations.
2. Description of Related Art
Methods for ferrous metallurgy are known, perhaps the most common method being the production of steel. Typically, iron ore and other various raw materials such as coke, limestone and dolomite are heated in a blast furnace to a sufficient temperature to melt the raw materials and allow them to mix. Slag is separated from the mixture and the remaining molten metal is transferred to a steel melting shop where further refining is done. The resultant crude steel can then be further refined with the addition of alloys that give the particular steel the desired properties. As is known, some of the above processes can be supplemented with the inclusion of scrap steel or iron. The resultant product is typically continuously cast into billets, blooms or slabs, sometimes referred to as “semis”, and these semis are then processed to form the final product. In some plants the product is cast directly into strip on strip casters. In others, the semis can be beam blanks or near-net-shapes to reduce rolling requirements.
During the processing of semis, the semis are typically heated to a temperature sufficient to allow the semis to be worked, a typical such temperature being 1200 degrees Celsius. The semis are then processed by a rolling mill, the design of the rolling mill dependent on the desired shape of the finished product. The rolling mill, through the application of heat and pressure, forms the steel product. Thus, significant energy is used to shape the semis into the steel product.
Steel product, in a final form, can be a variety of shapes and configurations. Steel product includes, for example, flat rolled steel, steel strip, bars, beams, wires, rods, sheets, plates, bands, channels, tubes, pipes, tracks, and rails. If the steel product is a bar or a beam, for example, it may be stored in bundles. When steel product is shaped into flat rolled steel, for example, it is often rolled into round coils. Steel product, when shaped into wire or rod, for example, is also often typically rolled into round coils. For ease of reference, coils of steel product will also be referred to as bundles unless otherwise noted.
In general, there is a significant desire that the steel being produced have relatively constant dimensional straightness. Thus, significant resources are exerted in controlling the rolling mill process so that the finished product has the correct dimensions and straightness. Steel product with poor dimensional straightness control must be either sold at a lower cost, be reworked, or be reprocessed. The designation for out of tolerance straightness is referred to in the trade as camber or sweep; herein it will be called warp or warpage. Part of the process of producing steel product involves cooling the hot shaped steel to a temperature where the steel is dimensionally stable and/or can be stored. As is known, the rate at which steel cools has a significant affect on the properties of the steel due to, in part, the affect the rate of cooling has on the grain structure of the resultant steel product. Uneven cooling tends to produce stresses in the steel and such stresses may cause the steel product to warp or crack or otherwise suffer damage. When some coils are produced, it is necessary to retard the rate of cooling to prevent damage from stress. Special furnaces or other devices such as covers are used to control the rate of heat loss and temperature reduction.
A somewhat similar problem can be caused by hydrogen entrapment in the metal. When hydrogen is trapped in miniscule voids in the metal it can lead to a phenomenon known as hydrogen embitterment. This can result in localized weakness and cracking of the metal if the hydrogen is not removed. Hydrogen and other gasses are often removed using special degassing equipment. They can be vacuum, magnetic stirring or argon stirring. Stirring is used because the liquid metal surface has less head pressure and can more easily release the entrapped gasses.
Therefore, substantial resources are devoted to ensuring the hot shaped steel cools at a desired rate. Often the hot shaped steel is controllably cooled on a cooling bed. Cooling beds, depending on the dimensions of the steel product, and the desired rate of cooling, can be quite long and can add significant cost to the production of steel because of the upfront capital expenditures required to create the necessary facilities. Sometimes the size of the cooling bed is a limiting factor in determining the rate at which the steel production facility can operate. In addition, the time needed to cool the steel increases the amount of work in process. Naturally, increasing the amount of work in process increases the necessary level of inventory, which in turn decreases the efficiency of the plant operation. In addition, higher levels of inventory make the steel production facility less flexible and potentially less able to respond quickly to variations in the quality of the steel product. Thus, a decrease in the level of inventory would tend to make a steel production facility more profitable while potentially increasing the quality of the steel product produced.
For example, as is known in the art, when the steel product is a steel bar, the steel bars are first sufficiently cooled and then bundled together via straps and removed from the production line and typically placed in a storage facility until the steel product is transported to the customer. If the steel bars are bundled too soon, the interior portion of the bundle will cool at a slower rate than the exterior portion of the bundle. Also, the portion of the steel bar that is exposed to the outside air will cool more rapidly than the portion of the steel bar that is in contact with other bars. Thus, the exterior steel bars of the bundle will have internal stresses as a result of the disparate cooling rates. These stresses can cause the steel bars to warp once the straps holding the bundle together are removed, potentially making the steel bars unusable.
Longer cooling beds relieve this problem but, as discussed above, are costly and inefficient to implement. As can be appreciated, general storage facilities are somewhat less costly to install and maintain as compared to cooling beds. And the storage facilities are usually a necessary requirement anyway. Thus, storing the steel in a storage area while the steel cools would be less costly from a facility investment perspective and this decreased cost could significantly benefit the profitability of the steel production facility. Therefore, it would be beneficial to be able to bundle the steel bars sooner (i.e., while still quite hot) without having to later rework the steel bars due to warpage caused by internal stress concentrations affecting the dimensional straightness of the steel bars.
Once the steel product is delivered to the customer, the steel product is typically further processed to make finished goods. The processing can include machining the steel, drilling, punching, grinding, cutting, welding, cold working the steel, and various other known methods of processing steel into finished goods. During this process of working the steel, the initial internal forces are often unbalanced in the steel product. These forces tend to create localized stress concentrations in the finished good. As can be appreciated, a particular grade of steel can only withstand a particular level of stress before the steel deforms in an undesirable plastic manner. Thus, it is undesirable to have excessive internal stresses in the steel product prior to the steel product being processed into the finished good, because this additional processing can cause the internal stresses to distort the final product.
Depending on the desired properties, even the localized stresses created by the processing of the steel product into the finished good may be undesirable. Therefore, various methods of relieving the stresses of finished goods are known. One method is to let the finished good sit for a substantial time so that the excessive internal stress concentrations have time to relax. Another method is to heat the finished good so that the internal stress concentrations can more quickly be relieved. Another method is to vibrate the finished good in a known manner, the vibrations providing energy that allows the stress concentrations to more quickly dissipate. While these methods of reducing the resultant stresses in the finished product are sometimes necessary, it is undesirable for significant variations in the stress concentrations to exist prior to the processing of the steel. Therefore, it would be advantageous to ensure the steel product, before being further processed, is essentially free of internal stresses or at least has a relatively constant internal stress level throughout the steel product.